Electrical apparatus



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'RF INPUT -Rh UENCY INVHVTOR JULIUS HALPERN emilion iiUUlil Patented June 14,-, 1949 UNITED STATES PATENT OFFICE ELECTRICAL APPARATUS Julius Halpern, Brookline, Mass., assignor, by mesne assignments, to the United States ol.' America as represented by the Secretary of War Application September 6, 1945, Serial No. 614,727

3 Claims. 1

This invention relates generally to electrical apparatus and more particularly to a frequency analyzer for electromagnetic oscillations.

It is frequently desirable to determine experimentally the frequency components of complex electromagnetic oscillations. This may be done through the use of some type of selective circuit or circuit element which will respond substantially to only one frequency at a time. One type of circuit element possessing these selective characteristics to a high degree is a resonant cavity, since it is well known that any closed surface having conducting walls can support an oscillating electromagnetic field within. it and possesses resonance at certain frequencies when excited by electromagnetic oscillations. Cavity resonators have the advantages of simplicity, relatively large physical size, high Q, and very high shunt impedance. That type of cav- ,ity generally known as non-reentrant is typified in idealized form by the cylinder, prism, sphere, ellipsoid, etc., and possesses in general a very high Q and a resulting high selectivity. Resonant cavities of this type may be caused to scillate in such a mode that the theoretical Q is as high as 55,000. The resonant frequencies of a cavity of this type depend on its size and shape and may be adjusted by varying one dimension; for example, by moving one wall of the cavity in and out by means of a screw adjustment or other mechanical means.

It is therefore an object of the present invention to provide a frequency analyzing means employing a resonant cavity as a selective circuit element. It is a further object so to adapt this means that the resonant frequency of the cav- Y`ity may conveniently and periodically be varied through a known series of values and the output magnitude of each component of a given complex electromagnetic oscillation may be measured. It is a further object of this invention to provide a visual indication of the frequency and amplitude of the various components of such a complex wave.

Other objects, features, and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in conection with the accompanying drawing in which:

Fig. 1 is a block diagram of a system embodying the principles of the present invention;

Fig. 2 is a perspective view of a second type of resonant cavity which may be employed as a Reference is now made more particularly to Fig. 1, in which a variable resonant cavity I0 is excited by the R.F. voltage to be analyzed through coupling loop I2 which is the .termination of a coaxial transmission line I4. One side of cavity I0 consists, in the embodiment shown, of a stili diaphragm I6 forming a central portion and bordered by a flexible annular portion I1 of cloth, or any other flexible material, which allows diaphragm I6 to move with relation to the remainder of the cavity. Side I8 of cavity I0 may be moved in and Iout by means of adjusting screw I9.

Diaphragm I6 is mechanically coupled to a vibrator coil 20 which may comprise the voice coil of an electro-dynamic type of loud speaker having a magnetic field supplied by a magnet 22. Coil 20 is excited by an A.C'. voltage of some low frequency, for example 60 cycles, through a transformer 24, the primary coil 25 of which is supplied by a source of alternating voltage of the desired frequency. Transformer 24 has two secondary coils 26 and 28, the former of which is connected ln series with coil 20 and variable resistor 30. Secondary coil 28 is center tapped, and its two ends are connected to a phaseshifting network consisting of variable resistor 32 and capacitor 34 in series. The output of the phase-shifting network i-s obtained between the center tap of secondary coil 28 and the common terminal of resistor 32 and capacitor 34. This .output is applied as a deflection voltage to the horizontal deflection plates 36 and 38 of cathode ray tube indicator 40.

The output of resonant cavity I0 is coupled through coupling loop 42 and coaxial transmission line 44 to a crystal detector 46. The output of detector 46 is applied to a pulse stretching circuit consisting of capacitor 48 and resistor 50 and the output of this circuit in turn is applied to video amplifier 52. The output waveform supplied by video amplifier 52 is then applied to the vertical deflection plates 54 and 56 of cathode ray tube indicator 40.

Reference is now made more particularly to Fig. 2 showing a resonant cavity 60 of a second type suitable for use as part of the present invention. R.F. energy is coupled into the cavity through coaxial transmission line 62 and coupling loop 64. The volume of the cavity is varied by diaphragm 66, which is constructed wholly of one type of flexible material such as fiber, and may be attached by Coupling 68 to a vibrator coil such as described in connection with Fig. 1. The

part of the present invention showing an al- R.F. output is taken from the cavity through ternative method of varying the resonant frecoupling loop 'ln and coaxial transmission line quency of the cavity; and 12. Side 'I4 of cavity 60 may if desired be made Fig. 3 shows a typical frequency spectrum obmovable and adjusted by means of a screw adtained on the indicator of the preferred embodijustment similar to that described in connection ,ment ofthe present invention. with cavity I0 of Fig. 1.

When the apparatus of Fig. 1 is in operation, the input energy, which may be either a continuous wave or a series of pulses of R.F. oscillation, is fed into the cavity by means of coupling loop I2. If these oscillations have a frequency component of the frequency to which cavity III is resonant, an oscillatory field is set up within the cavity, the amplitude of which is proportional to the amplitude of the given frequency component. oscillations is coupled by means of loop 42 and coaxial line 44 to detector 46, the output of which then becomes proportional to the amplitude of the given frequency component. After slight modification by capacitor 48 and resistor` 50, the operation and purpose of which will be described later, the output of detector 46 is applied to video amplifier 52, and the resulting output is applied to the vertical deflection plates 54 and 56 of cathode ray tube indicator 40 to cause a vertical deflection of the electron beam of the tube of a magnitude proportional to that of the frequency component under consideration.

The low frequency A.C. voltage from secondary 26 of transformer 24 flowing in coil 20 causes a periodic motion of this coil of the same frequency as the applied voltage. This motion is transmitted to diaphragm I6 and causes a corresponding periodic variation of the resonant frequency of cavity I0. The maximum deviation of the resonant frequency during each cycle of diaphragm I6 may be controlled by varying the magnitude of the current through the coil by means of series resistance 30. The mean or center of frequency of the cavity may be shifted up or down by adjusting the position of side I8 by means of screw adjustmentl I9.

In order to cause a visible indication of the frequency spectrum of the input oscillation to appear on the screen of cathode ray tube 40, it is necessary to displace the electron beam laterally at the same frequency and in phase with the sinusoidal variation of the resonant frequency of the cavity. The sinusoidal voltage from secondary 2li-of transformer 24 is thus applied to horizontal deflection plates 36 and 38. To insure that the variations of the beam position and the resonant frequency are in phase, the network consisting of resistor 32 and capacitor 34 is inserted. The output of this network is of vconstant amplitude but may be varied through approximately 360 in phase by varying resistor 32. By proper adjustment of resistor 32 the beam of cathode ray tube 40 may be caused to be at the extreme left when the resonant frequency of the cavity is the lowest.

A typical indication of a frequency spectrum as it appears on the indicator of the system is shown in Fig. 3 with the frequency plotted horizontally and the amplitude of the various frequency components plotted vertically.

If the incoming electromagnetic oscillations are in the form of short pulses of R..-F. energy of the order of one microsecond in duration and separated by intervals of perhaps 1000 microseconds, the trace on the screen of the indicator becomes very faint. For this reason, a pulse stretching network consisting of capacitor 48 and resistor 59 is placed in the output circuit of detector 46. When a pulse is passed by detector 46, capacitor 48 is partially charged. After the termination of the pulse, it discharges through A portion of the energy of these resistor 50 and thus maintains a voltage of the same polarity as the pulse across the input terminals of video amplifier 52 for a short period. This pulse stretching effect serves to improve the visibility of the trace on the screen of the indicator.

While there has been described what is at pres- .ent considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made ,therein without departing from the scope of the invention as set forth in the appended claims.

The invention claimed is:

l. In combination, a resonant cavity, electromechanical means associated with said cavity for varying the resonant frequency of said cavity periodically at an audio rate, means for changing the maximum frequency deviation of said periodic variation, means for changing the mean frequency of said variation, means for coupling an electromagnetic oscillation into said cavity, detecting means coupled to the output" from said cavity, amplifying means coupled to the output of said detecting means, and indicating means, including a cathode ray tube, means for vertically deecting the beam of said tube in proportion to the instantaneous amplitude of the output from said amplifying means, and means for horizontally deflecting said beam in synchronism with the variation of the resonant frequency of said cavity.

2. In combination, a resonant cavity, means associated with said cavity for varying the resonant frequency of said cavity periodically, means for changing the maximum frequency deviation of said periodic variation, means for changing the mean frequency of said variation, means for coupling an electromagnetic oscillation into said cavity, detecting means, means for applying oscillatory energy from said cavity to said detecting means, amplifying means coupled to the output of said detecting means, and indicating means responsive to the output of said amplifying means for giving a visual indication of the instantaneous amplitude ofsaid output.

3. In combination, a resonant cavity, means associated with said cavity for varying the resonant frequency of said cavity periodically by producing a periodic variation of a dimension of said cavity, means for changing the amplitude of said periodic variation, means for coupling an electromagnetic oscillation into said cavity, and means coupled to said cavity for indicating visually the instantaneous amplitude of the electromagnetic oscillation present in said cavity and the instantaneous resonant frequency of said cavity.

JULIUS HALPERN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

